The Minnesota Study of Twins Reared Apart involves a comprehensive psychological and medical assessment of monozygotic and dizygotic twins who were separated early in life, reared apart, and reunited in adulthood. The long-term objectives of the study are: a) to evaluate the genetic and environmental factors that influence various traits, and b) to explore possible genotype-environment interactions in the determination of these traits. The medical and psychological traits selected for study are of broad interest to psychologists, physicians, behavioral geneticists, and the lay public.